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CHAPTER 5 Computing Stormwater Runoff Rates and Volumes

New Jersey StormwaterBest Management Practices ManualFebruary 2004 CHAPTER 5 Computing StormwaterRunoff Rates and VolumesThis CHAPTER discusses the fundamentals of Computing Stormwater Runoff Rates and Volumes from rainfallthrough the use of various mathematical methods. To do so effectively, the CHAPTER also describes thefundamentals of the rainfall- Runoff process that these methods attempt to simulate. Guidance is alsoprovided in the use of the Natural Resources Conservation Service, Rational, and Modified Rational Methodsthat are specifically recommended and/or required by the NJDEP Stormwater Management Rules at :8.

New Jersey Stormwater Best Management Practices Manual • Chapter 5: Computing Stormwater Runoff Rates and Volumes • February 2004 • Page 5-4 1. High intensity rainfall will generally produce a greater peak discharge than a rainfall that occurs

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Transcription of CHAPTER 5 Computing Stormwater Runoff Rates and Volumes

1 New Jersey StormwaterBest Management Practices ManualFebruary 2004 CHAPTER 5 Computing StormwaterRunoff Rates and VolumesThis CHAPTER discusses the fundamentals of Computing Stormwater Runoff Rates and Volumes from rainfallthrough the use of various mathematical methods. To do so effectively, the CHAPTER also describes thefundamentals of the rainfall- Runoff process that these methods attempt to simulate. Guidance is alsoprovided in the use of the Natural Resources Conservation Service, Rational, and Modified Rational Methodsthat are specifically recommended and/or required by the NJDEP Stormwater Management Rules at :8.

2 This guidance includes use of the methods to comply with the Rules groundwater recharge, stormwaterquality, and Stormwater quantity actual physical processes that convert rainfall to Runoff are both complex and highly variable. As such,these processes cannot be replicated mathematically with exact certainty. However, through the use ofsimplifying assumptions and empirical data, there are several mathematical models and equations that cansimulate these processes and predict resultant Runoff Volumes and Rates with acceptable selection of the appropriate model or equation depends upon a number of ResultsSome methods, such as the Rational Method, can be used to produce estimates of peak Runoff Rates , butcannot predict total Runoff Volumes .

3 Other methods, conversely, can only produce estimates of total runoffvolumes, while others, such as the Natural Resources Conservation Service (NRCS) methods, can accuratelypredict both total Runoff volume and peak rate, and even entire Runoff Area SizeDue to their assumptions and/or theoretical basis, some methods can accurately predict Runoff Volumes orrates only for single drainage areas of 20 acres or less, while other methods can be applied to watersheds of20 square miles or more with 100 or more Jersey Stormwater Best Management Practices Manual CHAPTER 5: Computing Stormwater Runoff Rates and Volumes February 2004 Page 5-2 Data AvailabilitySimple methods, such as the Rational or Modified Rational Methods, require limited rainfall and drainagearea data, while other, more sophisticated methods have extensive data needs, including long-term rainfalland temperature data as well as drainage area soils, subsoil, and ground cover information.

4 In general, themore data-intensive models can produce more comprehensive Runoff general, Stormwater Runoff can be described as a by-product of rainfall s interaction with the land. Thisinteraction is one of several processes that the earth s water may go through as it continually cycles betweenthe land and the atmosphere. In addition, Stormwater Runoff is only one of many forms water may takeduring one of these cycles, known scientifically as the hydrologic cycle. Shown in Figure 5-1 below, thehydrologic cycle depicts both the primary forms that water can take and the cyclical processes that producethem.

5 In addition to Runoff , these processes include precipitation, evaporation from surfaces or theatmosphere, evapotranspiration by plants, and infiltration into the soil or groundwater. As such, water thatprecipitates as rainfall can wind up or at least spend time on ground or plant surfaces, in the atmosphere,within the various soil layers, or in waterways and water 5-1: The Hydrologic CycleSource: Fundamentals of Urban Runoff general, all Runoff computation methods are, to some degree, mathematical expressions of thehydrologic cycle. However, most transform its cyclical character to a linear one, treating rainfall as an inputand producing Runoff as an output.

6 During this transformation, each method uses mathematicalapproximations of the real rainfall- Runoff processes to produce its estimates of Runoff volume and/or rate. Asdescribed above, each method has its own complexity, data needs, accuracy, and range of the key input, rainfall is generally characterized by its size, intensity, and the frequency of itsoccurrence. The size of a rain storm is the total precipitation that occurs over a particular duration. HowNew Jersey Stormwater Best Management Practices Manual CHAPTER 5: Computing Stormwater Runoff Rates and Volumes February 2004 Page 5-3often this size of storm is likely to reoccur is called its recurrence interval.

7 For instance, a rainfall of certainduration that occurs, on average, once every 25 years would have an average recurrence interval of 25 yearsor be called a 25-year storms have been shown to be mathematically random events, their recurrence can also bespecified as an annual probability. The equation for converting between recurrence interval and annualprobability is:Annual probability (in percent) = 100/recurrence interval (in years)For example, the 25-year storm noted above could also be described as having a probability of 4 percent(=100/25) or a 4 percent chance of being equaled or exceeded in any given year.

8 Similarly, a 2-year stormhas a 50 percent chance (=100/2), a 10-year storm has a 10 percent chance (=100/10), and a 100-yearstorm has a 1 percent chance (=100/100) of being equaled or exceeded in a given year. Resultant runoffpeak Rates and Volumes events can also be described in such Volumes are influenced primarily by the total amount of rainfall. However, Runoff Rates resultingfrom a given rainfall, including the peak rate or discharge, are influenced primarily by the rainfall sdistribution, which is how the rainfall rate or intensity varies over a period of time.

9 Studies of rainfallrecords show that actual storm distributions and durations can vary considerably from event to event. Arainfall may be evenly distributed over a time period or can vary widely within that same period. Itsduration can also be long or very short. These different types of rain events can produce extremely differentrunoff Volumes and peak computation methods deal with this rainfall variability in one of two general ways. Manymethods, including the Rational and NRCS methods, rely on a hypothetical rain event known as a designstorm for their rainfall input.

10 This single, hypothetical storm event is based on a compilation of local orregional rainfall data recorded over an extended time period. To use a design storm, the user must makesome assumptions about the antecedent ground and waterway conditions that exist at its start. Most runoffcomputations are based on average antecedent conditions, although wetter or drier conditions can also beused depending upon the user s interests and of compiling long-term rainfall data into a single design storm, other Runoff computationmethods address the variability of real rain events by analyzing a long series of them, Computing Runoff rateand volume estimates for each.


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